Materials for constructions and a method for producing same

The present invention relates to materials for constructions and a method for producing same. In particular, the invention relates to low energy absorbing aluminium sheet materials for buildings or other appropriate applications such as in transportation vehicles.

In this particular technical field, the terms emittance or emissivity (ε) are referring to the ability of a surface to irradiate (emit) electromagnetic radiation. Solar reflectance (or reflectivity) refers to the ability of a surface to reflect solar radiation.

The reflectance (p) and emittance (ε) of a surface can vary strongly with wavelength (λ) of radiation. The emittance (ε) of an object is defined as the ratio of radiant energy emitted by the object to that of a perfect Planckian blackbody radiator at the same temperature and wavelength, that is, an obje.et following Planck's law. Thermal emittance is determined by a weighing process of the emittance, ε(λ), in the thermal wavelength region of the electromagnetic spectrum.

For opaque materials with no transmittance, the relation between emittance and reflectance simplifies to the following: ε(λ) = 1 -p(λ)

Solar radiation contains a significant amount of energy. When direct sun radiation is absorbed in a structural element the temperature increases.

Solar heat gain will increase the amount of excessive heat that is transported into the building interiors. Such absorption of solar energy and heat transfer through building materials may have a negative impact on the cooling load and personal comfort during the warm season.

Prior art solar reflective coatings

The solar reflectance of a surface is the fraction of the incident solar energy, which is reflected by the surface in question. The best standard technique for its determination uses spectrophotometric measurements with an integrating sphere to determine the reflectance at each wavelength. The total solar reflectance (TSR) is determined by a

weighing process, using a standard terrestrial solar spectrum (see figure 2). ASTM E903 and E892 document this method.

The solar spectrum consists of three wavelength-regions and the energy is distributed approximately as follows:

Ultra violet region < 400 nm 5%

Visible region 400 - 700 nm 44%

Near infrared region 700 - 2500 nm 51 %

Therefore, to reduce solar heat gain, the reflective surface properties in near infrared region, as well as the visible and UV regions are of importance. The reflectance in the visible region will determine the visual appearance (colour and brightness) of the surface. Traditionally, solar heat gain is most problematic for black (or other dark coloured) surfaces. A black surface typically has a low reflectance in all parts of the solar spectrum. However, it is possible to produce a black surface with a much higher solar reflectance by altering the reflectance properties in the UV and near infrared parts of the solar spectrum. Since these parts of the spectrum are not visible to the human eye, such a modification of the surface reflectance properties will not alter the visual appearance of the surface.

Traditionally only white and light shade coloured coatings have given a relatively high solar reflectance. Special near infrared reflective coatings are a recent development that has found a main application as outside coatings on metal roofs. These paints keep the roof relatively cool, and allow at the same time a wide choice of roof colours. With near- infrared-reflective pigments, even the solar reflectance of black coatings has been increased to more than 25%, whereas conventional black coatings typically give a solar reflectance of only 5%.

Prior art low emissive coatings Low emissive coatings are known to have been applied to the surface of building elements for improved thermal performance. One example is the application of low emissive coating on the inside of metal roofs. These types of low emissive coatings were initially developed for military applications. The low emissive surface of, for example a military vehicle, can alter and suppress the thermal radiation from the object and make it harder to detect with infrared sensors. Such low emissive coatings are typically produced with the use of metallic pigments.

Aluminium has a high reflectance in the thermal region of typically 0.9; corresponding to a thermal emissivity of 0.1 , see Figure 1. For this reason, metallic aluminium flakes are commonly used as pigments in such low-e coatings.

Aluminium building products are usually surface treated in order to yield an appropriate durability and appearance, without particularly addressing the emissive or solar reflective properties. Further, the surface treatment needs to comply with the standards in the market field (eg. EN 1396, GSB International, Qualicoat or Qualanod). For most surface treatments it is very difficult to combine outdoor durability, scratch resistance, acceptable visual appearance, acceptable production cost and other standard requirements with low emissivity. Normally the metal is coil coated, anodised or powder coated. This gives excellent outdoor durability but high emissivity.

Standard surface treatment procedures applied today typically result in thermal emissivities of ε=0, 85-0,9 on all surfaces. According to the invention it is possible to improve the thermal performance of the said structural metal element by applying coatings with optimised optical properties on the various surfaces, while at the same time fulfilling the EN 1396, GSB and/or Qualicoat and/or Qualanod requirements.

The applicants ownpatent application PCT/NO2005/000224 relates to an element with improved thermal properties. The improvements are based upon the fact that the thermal emittance as well as the solar reflectance of the various surfaces will influence the thermal properties of a structural metal element. Further, the desired solar reflective and thermal emissive properties of the exterior surface can be different from that of the internal and interior surfaces.

In accordance with that application, a structural metal element with improved thermal properties can be made by improving the optical properties (emissivity and solar reflectance) of the various surfaces. The internal and interior surfaces should preferably have a low thermal emittance. The exterior surfaces should preferably have a high solar reflectance. The thermal emittance of the exterior surface is of less importance due to larger heat transfer by convection at the exterior surface.

Further, that invention describes three different procedures to achieve such improved optical properties for an element consisting of two (one inner and one outer) or more separated metal sections.

In accordance with the present invention there now can be made materials for constructions with improved thermal properties. In particular the improvements relates to structural elements of aluminium or an aluminium alloy. The structural element can for instance be represented by sheets as facade panels for wall- or roof- applications for buildings or transportation purposes. The improvement can result in lower inside temperature in buildings during summertime, with reduced energy consumption by air- condition equipment etc.

The above mentioned and further advantages can be achieved in accordance with the invention as defined in the accompanying claims.

The invention shall be further explained by examples and figures where:

Figure 1 : Reflectance of aluminium in the wavelength region 0,3 - 50μm

Figure 2: Solar energy as a function of wavelength

Figure 3: Surface temperature and emitted temperature for different materials. Emitted temperature is measured on "unmasked" surface and surface temperature is measured on a tape adhered to said surface.

The reflectance of a surface can change depending on the wavelength of the incoming radiation. For practical applications in buildings we can look at the reflectance properties in the solar region (0,3 - 2,5μm) and the thermal region (2,5 - 50μm) in the electromagnetic spectrum. See figure 1.

The temperature of a surface will increase when exposed to direct sunshine. The temperature of a standard black surface can reach a temperature of 70°C. To reduce the surface temperature, new pigments have been developed which reflects in the near infrared (NlR). The NIR region has 51% of the solar energy. (See figure 2). By reflecting in the solar region the surface temperature will be lower than standard colours.

The reflection properties in the thermal region of the spectra control the emissivity. The emissivity controls the radiated heat from a surface. Aluminium metal has an emissivity of

0,01. Any coat on an aluminium surface will reduce this emissivity:

• Anodising 20μm: 0,85

• Anodising 5 /vm: 0,85

• Anodising 1 μm: 0,2 • Clear lacquer 5μm: 0,9

• Clear lacquer 2μm: 0,3

• Sol-gel coating 0,5μm: 0,16

It has been observed that a metal material coated with an exterior coating with high total solar reflection and a high emissivity in combination with an interior coating with low emissivity gives the lowest heat gain.

The present invention is to have an aluminium substrate with the same exterior paint as competing steel substrates, but on the reverse side we will apply a sol-gel coating and we will have an emissivity of 0,15 - 0,20 depending on the film-thickness of the sol-gel coating.

There has been performed a test where it has been measured surface temperature and emitted temperature on 3 different materials, see figure 3. All samples are coated with the same Reflective Dark Grey on the exterior side and the coatings on the reverse and interior side are different: 1 is bare aluminium with emissivity of 0.1; 3 is a coil coating aluminium metallic coating with an emissivity of 0,5 and 5 is a low emissive coating developed for steel with an emissivity of 0,5. The surface temperature is measured by adhering a tag of tape to the surface, whereby an infrared sensor apparatus is reading the temperature of said tape.

The emitted temperature is measured by similar temperature measurement reading the emitted temperature of an unmasked part of said surface.

These measurements have been performed when the samples have been exposed for direct sunshine behind a window glass to avoid influence of wind.

Reducing the emissivity of the reverse side from 0.5 to 0.1 has reduced the emitted temperatures from 37° to 27 ⁰ C.